a(n) = 4^n + 1.

Terms

a(0) =2a(1) =5a(2) =17a(3) =65a(4) =257a(5) =1025a(6) =4097a(7) =16385a(8) =65537a(9) =262145a(10) =1048577a(11) =4194305a(12) =16777217a(13) =67108865a(14) =268435457a(15) =1073741825a(16) =4294967297a(17) =17179869185a(18) =68719476737a(19) =274877906945